Conventionally, rotors of interior permanent magnet motors configured as described below have been proposed. Specifically, the rotor of the interior permanent magnet motor includes a rotor core that is formed by laminating a plurality of magnetic steel sheets; permanent magnet insertion holes that are formed in an axial direction of the rotor core and are formed in portions corresponding to the sides of a substantially regular polygon centered on the axial center; permanent magnets that are inserted into the permanent magnet insertion holes; a plurality of slits that are formed in a core portion on the outer circumferential side with respect to the permanent magnet insertion holes and are spaced apart along the permanent magnet insertion holes; and outer-side thin portions that are provided between outer side ends of the slits in a radial direction and an outer circumference of the rotor core and each gradually increases its width in the radial direction toward interpolar portions from a magnetic pole center. With such a configuration, the harmonics of the induced voltage and cogging torque can be reduced by reducing the harmonic components of the magnetic flux density waveform in the interpolar portions (for example, see Patent Literature 1).